Photo-cell pickup system



I March 1940- I c. H. BRASELTON 2,193,789

PHOTO -CELL PICKUP SYSTEM Original Filed Jan. so, 1936 2 Shets-Sheet 1 c f rjigaxw ATTORNEYS.

March 19, 1940. c. H. BRASELTON 2,193,789

' PHOTO-CELL PICKUP SYSTEM Original Filed Jan. so, 1936 ZShgts-Sheet 2 I f2 ,7 I fiIMIM-J Patented Mar. 19, 1940 UNITED STATES I 2,193,7se PATENT OFFICE PHOTO-CELL PICKUP SYSTEM Chester H. Braselton, New York, N. Y., assignor to National Television Corporation, Wilmington, Del, a corporation of Delaware Application January 30, 1936, Serial No. 61,441 Renewed August 17, 1939 10 Claims.

This invention relates to photo-cell pickup systems, and more particularly to new and improved arrangements of photo-cell circuits, to provide larger changes in current for given changes in light'intensity, and to respond with .a given change in current intensity to smaller changes in light intensity than heretofore.

It is an object of this invention to provide such a system which 'may be utilized for television pickup under conditions where satisfactory pickup has heretofore been difiicult or impossible of achievement.

It is a still further object of my invention to provide an arrangement in which relatively large photo-cell currents may be obtained under control of relatively small changes of light intensity.

. Another object of the invention is to provide 1a regenerative circuit for a photo-electric cell.

, A further object of this invention is to provide a television pick-up system having greater flex- .ibiIit'y and utility than systems heretofore known.

Still other objects and advantages of my in vention will be apparent from the specification.

' My invention itself, however,- both as to its fundamental principles and as to its particular embodiments, will best be understood by reference to the specification and. accompanying drawings, in which:

Fig. 1.is a circuitdiagram of a television pickup in accordance with my invention;

Fig. 2 is a circuit diagram of a modified form thereof;

Figs. 3 and 4' are circuit diagrams of two other modified forms of the invention; and

Fig. 5-is a circuitdiagram of still another modification of the invention.

Referring now more particularly to Fig. 1, l

designates a photo-electric cell having a cathode {and an anode 3. A resistance 1 and a source of potential H are connected in'series and across the output terminals of the cell. The potential difierence developed across resistance 1 is applied to the control circuit of an amplifier, herein shown in its simplest form as a vacuum tube 8,

:having cathode 9, control electrode l0 and anode IL. For simplicity, the filament heating cir- --cuithas been omitted.

Connected in .the output of the amplifier 8 I may provide resistance 13 and gaseous discharge lamp M. The voltage drop through a suitable portion-of the output circuit of amplifier 8 may .be impressed upon the succeeding amplifier,

herein diagrammatically illustrated by vacuum .tube 20. It will be understood that the tube indicated as 26 may, in fact, be a series of amplifier stages feeding the final output terminals, which may be connected tothe modulation input of a transmitter.

' nssociated'wit'h the photo-cell I, Imay provide any suitable form of scanning apparatus for projecting upon I the photo-cell the elemental light values corresponding to elemental picture areas. The scanning apparatus may be of any type desired, and by way of example, I have shown a motor l6 driving a scanning disc H with a suitable optical system, such as lenses l8 and 2|, for projecting an image upon the scanning disc, and thence after the scanning action,-on to the cathode of the photo-electric cell i.

The lamp M referred to as a gaseousdischarge lamp, may be arranged to project its light also upon the cathode 2 of the photo-electric cell .i through a suitable optical system H), as indicated in the figure.

number of types, such as the crater'lamp, in

which the discharge is concentrated in a cavity in one of the electrodes, or the plate electrode lamp, in which the discharge forms a glow upon the cathode, and it may contain neon or other ionizable gases, at a reduced pressure sufiici'ent to give the desired result. v The glow of such a lamp will be modulated in accordance with the variation of potential across its electrodes and as connected in the plate circuit of the amplifier tube 8, the light of lamp Hi will be proportional to the output of the tube.

This lamp M may be one of a The arrangements may be termed an optical I feedback and operates in the following manner: The image of thescene desired to be scanned is projected upon the scanning apparatus and thereafter upon the cathode 2 of the photoelectric cell i, in any suitable manner, producing, as will be understood, fluctuations in the control photo-cell current, which fluctuations correspond to the intensity of the light falling upon fluctuations of current produced by the television a scanning apparatus.

, The effect is to produce a considerably greater photo-electric current than could be produced by the original light only falling upon the photocell 1, but at the same time, controlled by the fluctuations of the light and in phase therewith.

Thus a regenerative effect is obtained,,the output of the cell i being fed back into the cell again in the form of light. The arrangement is very useful especially in television pickup devices as it makes it possible to obtain a fairly heavy output from a photo-electric cell when there is only a small amount of light available from the television pick-up device to influence the cell. It also has the advantage of permitting positive control of the regeneration of the circuit in a very convenient manner, as the amount of light falling on the cell from the lamp 14 may be easily controlled, as, for instance, by altering the focus of the light upon the negative electrode of the cell by adjusting the position of the lens IS.

The cell is diagrammatically indicated as having one light-sensitive electrode upon which the light from the scanning device, as well as the light from the lamp l l, falls. However, it may be desirable to provide two separate electrodes, one to receive the light from the scanning device and the other to receive light from the feedback exciting lamp. Such an arrangement is shown in Fig. 2. In this figure the photo-cell envelope 22 contains the two light-sensitive electrodes 23 and 24 which are connected together as indicated, the electrode receiving the light from the scanning disc ll and the electrode 24 receiving the light from the lamp E4. The remaining wiring may be exactly the same as shown in Fig. 1.

This arrangement of providing two lightsensitive electrodes has certain advantages, among which may be mentioned the fact that the electrode may be given certain characteristics for optimum performance under the conditions of use. For instance, this electrode may have its characteristics chosen so as to respond to very small changes of light intensity, While the electrode 24 may be chosen to respond to relatively large changes of light intensity or to the particular color of the light emitted by the lamp l4. Thus, the arrangement may be utilized to produce relatively large changes of photo-cell current under conditions such that the light coming from the scanning device would normally produce insufficient output current for the amplifier.

While I have referred to amplifier 3 as being a vacuum tube, it will he understood that this is diagrammatic, and this amplifier may be made up of a number of stages to provide the desired current flow through lamp i l, and since the lamp has substantially zero thermal inertia, it will be understood that this lamp will follow exactly the fluctuations of light intensity.

In Fig. 3 I have shown a modification of the invention 01 Fig. l in which I use a first cell 25 which receives the light from the scanning device and may be called the control cell. It has a light sensitive electrode 25 and a cooperating electrode 2?, and may be similar in every respect to the cell i of Fig. 1. An additional independent photo-eiectric cell 2* having a light sensitive electrode 29 and a cooperating electrode 35 may be used to receive the light of the lamp hi and may be called the booster cell. The cells 25 and 23 may be connected in parallel, as indicated, and the other electrical connections may be xactly the same as shown in Fig. l. The light from the scanning device may be directed to the electrode 26 of the cell 25, while the additional light from the lamp i l may be directed upon the light-sensitive electrode 29 of the cell 28. This arrangement has the advantages of that shown in Fig. 2, where the double electrode photo-cell is used, and also the additional advantages that the characteristics of the two cells may be made more extreme than where the two light-sensitive electrodes are in the same envelope. Thus, different gases may be used in the two cells, or one may be a vacuum cell and the other a gas cell, or other differences designed to meet certain desired requirements may be incorporated. A further advantage is that the wiring of the whole circuit may be shortened and more conveniently arranged as the light source l4 and photo-cell 28 may be placed in a separate enclosure entirely apart from the photo-cell 25.

Under certain conditions, in television, it may be desired to provide greater current fluctuations with increase in light frequencies. This may readily be done without changing the characteristics of the amplifier, by utilizing the arrangement shown in Fig. 4. This arrangement may be exactly the same as that shown in Fig. 3 except that an inductance 3| may be connected as a shunt by-pass around the lamp H. The inductance 3! may be so designed that its impedance increases with increase of frequency. At lower frequencies its impedance may be very low, and a large proportion of the current may be lay-passed through it. As the frequency increases, however, less and less current will be by-passed, and the average illumination of the lamp M will increase, and the amount of boosting action obtained from lamp M will increase with frequency.

In Fig. 5 I have shown a modified form of the invention where the light from the feed-back exciter lamp not only boosts the signal from the television apparatus but may also be used to maintain the average light from the television scanner at a more constant value. this figure the photo-cell 32, provided with the light-sensitive electrode 33 and the cooperating electrode acts in exactly the same manner as the cell i of Fig. l, and the scanning disc ll and associated parts, as well as the amplifying tubes 3 and 29, perform the same function as with the circuit of Fig. l. A lamp 38, which may be similar to the lamp M of Fig. 1, may also be provided and may act to feed back certain light variations to the photo-cell 32. However, I provide an additional photo-electric cell 31 which I may mount in a position to receive light from the lamp 36 with a lens 38, if desired, to focus the light upon the cell. This cell 31 may also be connected across the resistance 1, as is the cell 32, but in a reverse direction, so that the positive electrode 39 is connected to the negative electrode 33 of the cell 32, while the negative electrode 40 is connected to the opposite end of the resistance '1, an additional battery {ll being provided to maintain the necessary potential on the electrodes. I may also provide in the circuit of this photo-cell 3'! an iron-core choke 42 which may be so chosen that it will prevent the rapid fluctuations of the television signal from causing corresponding fluctuations of the photo-cell output, but will pass a more or less constant current determined by the average light of the lamp 36.

The operation of the arrangement of Fig. 5 is as follows: The photo-cell 32 responds to the light from the television apparatus and from the discharge lamp 36 in accordance with the operation described in connection with Fig. l. The additional photo-cell 31, being connected reversely across the resistance 7, tends to decrease the potential on the grid of the tube 8 when the action of the photo-cell 32 tends to increase it, but this action, because of the choke 55, is not rapid enough to cancel the television signal,

which of course is varying at a very high frequency. It does, however, compensate for extreme variations of the average light of a picture. To make this point clear, if it is assumed that the scene being televised has a certain average light value which suddenly changes, owing to a change of background or of movement in the scene, to one in which the average light is very much brighter, then the output of the photo-cell circult, under, the heretofdre known practice, would be'greatly increased, to such an extent, perhaps, as to greatly interfere with the amplification necessary throughout the system. Under such a condition, using the present invention, the additional photocell 3i with its choke d2 will maintain the input to the amplifier d at or near its previous value, so that the amplifiers and other parts of the apparatus operate at optimum efficiency at all times.

It is to be understood that this additional photo-cell 31 may also be used with any of the arrangements shown in Figs. 2, 3, and 4, 01' in fact it may be used in connection with an ordinary photo-cell circuit, receiving its light directly from the scanning device without using the booster lamp.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of my invention, as will be understood by those skilled in the art.

What I claim is:

1. In a photo-cell pickup system, in combination, means for converting light fluctuations into electrical currents, means for amplifying said currents, means for producing from said amplified currents additional light fluctuations corresponding to the original light fluctuations and in phase therewith, and means variable in a predetermined manner with frequency for causing said additional light fluctuations to produce increased electrical currents corresponding in frequency to the original light fluctuations.

2. In a photo-cell pickup system, in combina-,

tion, means for converting light fluctuations into electrical currents, means for amplifying said currents, and means variable in a predetermined manner with frequency for feeding from the output of said amplifier into the input side there of electrical currents corresponding in frequency to said light fluctuations.

3. In a photo-cell pickup system, in combination, a photo-electric cell exposed to light variations to be converted into electrical currents,

means associated with the output of said photoelectric cell for amplifying said currents, a lamp having substantially zero thermal inertia positioned to shine on said photo-electric cell, means for energizing said lamp from the output of said amplifier, and means for by-passing amplified currents around said lamp, said means having an impedance increasing with frequency.

4. In a photo-cell pickup system, in combination, a photo-electric cell exposed to light variations to be converted into electrical currents,

means associated with the output of said photoelectric cell for amplifying said currents, a second photo-electric cell connected in parallel with the first photo-electric cell, and a lamp having substantially zero thermal inertia adapted to shine on said second photo-electric cell and energized by said amplifying means.

5; In a photo-cell pickup system, in combination, a photo-electric cell exposed to light variations to be converted into electrical currents, means associated with the output of said photoelectric cell for amplifying said currents, a second photo-electric cell connected in parallel with the first photo-electric cell, a lamp having substantially zero thermal inertia positioned to shine on said second photo-electric cell, said photoelectric cells having different characteristics.

6. In a photo-cell pickup system, in combination, a photo-electric cell exposed to light variations to be converted into electrical currents, means associated with the output of said photoelectric cell for amplifying said currents, a secamplifier, and means for by-passing currents around said lamp, said means having an impedance increasing with frequency.

7. The method of reinforcing light fluctuations which comprises producing a photo-electric current corresponding to the light fluctuations to be reinforced, amplifying said photo-electric currents, producing from said amplified currents supplementallight fluctuations corresponding to and in phase with the original light fluctuations, and varying said supplemental light fluctuations in a predetermined manner with frequency.

8. The method of obtaining relatively large photo-electric current fluctuations from relatively small fluctuations of light intensity which comprises converting said light fluctuations into photo-electric currents, amplifying said photoelectric currents, converting said amplified photoelectric currents into amplified light fluctuations, varying said light fluctuations in a predetermined manner with frequency, and converting said amplified light fluctuations into amplified photoelectric current fluctuations.

9. The method of obtaining relatively large photo-electric currenlt fluctuations from relatively small fluctuations of light intensity which comprises converting said light fluctuations into photo-electric currents, amplifying said photoelectric currents, converting said amplified photoelectric currents into amplified light fluctuations, varying said light fluctuations in a predetermined manner with frequency, converting said amplified light fluctuations into amplified photoelectric current fluctuations corresponding in phase to the original photo-electric current fluctuations, and combining said original and amplified photo-electric currents.

10. In a photo-cell pickup system, in combination, a photo-electric cell exposed to light varia- CHESTER H. BRASELTON. 

